Chap. 10.] Rainbow explained. 279 



called a fecondary or falfe bo\v, becaufe it is faincer 

 than the other j and what is moft remarkable is, that 

 in the falfe bow the order of the colours appears al- 

 ways reverfed. 



In the true or primary bow, we have feeh that the 

 rays of light arrive at the fpectator's eye after two re- 

 fractions and one reflexion ; in the fecondary bow, 

 the rays are fent to our eyes after two refractions end 

 two reflections, and the order of the colours is reverf- 

 ed, becaufe in this latter cafe the light enters at the 

 inferior part of the drop, and is tranfmitted through 

 the fuperior. Thus (Fig. 55.) the ray of light which 

 enters at B is refracted to A, whence it is reflected to 

 P, and again reflected to W, where, furTering another 

 refraction, it is fent to the eye of the fpectator. The 

 colours of this outer bow are fainter than thofe of the 

 other, becaufe, the drop being tranfparent, a part of 

 the light is tranfmitted, and confequently loft, at each 

 reflection. 



The phenomenon aflumes a femicircular appear- 

 ance, becaufe it is only at certain angles that the re- 

 fracted rays are vifible to our eyes. The lead refran- 

 gible, or red rays, make an angle of forty- two de- 

 grees two minutes, and the moft refrangible or violet 

 rays an angle of forty degrees feventeen minutes. 

 Now if a line is drawn horizontally from the fpecta- 

 tor's eye, it is evident that angles formed with this 

 line, of a certain dimenfion in every direction, will 

 produce a circle, as will be evident by only attaching 

 a cord of a given length to a certain point, round 

 which it may turn as round its axis, and in every 

 point will defcribe an angle with the horizontal line 

 of a certain and determinate extent. 



Let H O, for inftance (Fig. 53.) repre'fent the ho- 

 rizon, B W a drop of rain at any altitude, S B a line 

 T 4 drawn 



